The invention relates to an R.F. circuit arrangement comprising insulating substrate means, at least one R.F. transmission line comprising a ground plane on the substrate means, and two diodes respectively D.C.-connected to two portions of the ground plane. For D.C.-biasing of the diodes, the circuit arrangement further comprises slot means in the ground plane to mutually D.C.-isolate the two portions of the ground plane.
Such an R.F. circuit arrangement is known from U.S. Pat. No. 3 939 430. This patent discloses a single-balanced mixer the R.F. circuit of which includes three transmission lines utilizing a conducting film on a first major surface of a dielectric substrate, namely a slot line, a coplanar line (sometimes known as a coplanar waveguide), and a microstrip line. The conducting film forms a ground plane in which the slot line is defined, and forms both the central conductor and the outer, ground conductors of the coplanar line, the elongate central conductor being separated by a circumferential gap from the transversely-opposed ground conductors. The conducting film also forms the ground plane of the microstrip line which further comprises a strip conductor on the other major surface of the substrate. One end of the slot line is coupled to one end of the coplanar line, the slot and gap of the respective lines together forming a continuous conductor-free region. The other end of the coplanar line is coupled to one end of the microstrip line, the central conductor of the coplanar line being connected by a conductive pin extending through the substrate to the strip conductor of the microstrip line. At the junction of the slot line and the coplanar line, two diodes are connected in opposite electrical polarities between the central conductor of the coplanar line and, respectively, two regions of the slot line ground plane lying respectively on opposite sides of the slot. This junction arrangement operates as a 180.degree. hybrid junction (see also, for example, U.S. Pat. Nos. 3 678 395 and 4 032 849, and the paper "A 26.5-to-40 GHz Planar Balanced Mixer" by U. H. Gysel, Proceedings of the 5th European Microwave Conference, September 1975, pages 491-495).
In operation, a received R.F. signal is supplied to the junction by the slot line, while the microstrip line and the coplanar line serve to supply a local oscillator (L.O.) signal to and extract an intermediate frequency (I.F.) signal from the junction. A constructed embodiment described in the patent operated with an R.F. signal frequency of 9.4 GHz and an L.O. signal frequency of 7.8 GHz, giving an I.F. signal frequency of 1.6 GHz.
To permit the diodes to be D.C.-biased and hence to reduce the power of the L.O. signal with which a particular value of conversion loss is obtainable, a respective portion of each of the two regions of the ground plane that lie on opposites sides of the slot line (the portions including the connections to the diodes) is isolated from the remainder of the ground plane by contiguous slots which extend from the slot line to the coplanar line so as to define therewith a rectangular bias pad (see FIG. 6 of U.S. Pat. No. 3,939,430). Since the diodes are connected in opposite electrical polarities, they appear in series between the two bias pads and can thus be forward biased by the application of a suitable direct voltage between the pads.
To provide a path between each of the bias pads and the respective adjacent ground plane region for the received R.F. signal and the L.O. and I.F. signals, a plurality of beam-leaded coupling capacitors are connected therebetween. However, currently-available commercial capacitors of this kind with suitable capacitances have a significant inductance which makes them unsuitable for use at frequencies above 5-10 GHz since they can then present a substantial impedance in the paths of these signals and/or can exhibit resonant effects. Furthermore, the additional slots provided to D.C.-isolate the bias pads form a secondary transmission line, and in order to inhibit the coupling of R.F. energy out of the slot and coplanar lines into this secondary line, it is important for respective capacitors to be accurately positioned across the secondary line immediately adjacent the junctions of that further line with the slot line and with the coplanar line, as depicted in FIG. 6 of U.S. Pat. No. 3,939,430. Provided that each capacitor presents a low impedance across the secondary line, the line will reflect the majority of the R.F. energy that would otherwise be coupled into it. However, if either capacitor is not correctly positioned (in which case a length of the further line can act as a stub) or if either capacitor does not present a low impedance (in which case R.F. energy can propagate along the further line past the capacitor), the further line can be coupled to a significant extent to the main R.F. circuit and can cause undesired effects therein.
It may be noted that a somewhat similar circuit arrangement to that mentioned above is disclosed in U.S. Pat. No. 4 118 670.